Finishing that novel on a lit screen can mess with your circadian rhythms, scientists say.

Reading before you drift away to sleep may lead to a poorer night's rest—if you read on lit screen such as an iPad, researchers say.

The body's circadian rhythms depend largely upon cues such as light, which is part of the reason why crossing time zones can lead to jet lag. For instance, dim light in the evening typically leads the body to generate melatonin, a brain chemical that regulates sleep. Ninety percent of Americans now use some type of electronics at least a few nights per week within an hour before bedtime, and researchers wanted to know what kind of effect all those glowing screens have on our sleep patterns.

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The scientists compared the quality of sleep for 12 volunteers after they used an iPad in dim light before bed for five consecutive days. They compared that to volunteers who read an old-fashioned paper book before bed for five consecutive days.

According to the study, published today in the Proceedings of the National Academy of Sciences, using electronic devices suppressed melatonin levels. The participants took nearly 10 minutes longer to fall asleep, experienced a significantly lower amount of REM sleep, and reported less wakefulness in the morning.

It's not just that the extra light messes with us, the researchers say. It's also the quality of light: They point out that electronic devices emit light that is bluer than natural light, and earlier research suggests bluer light may have a greater impact on sleep and circadian rhythms.